1. Field of the Invention
The present invention relates to a radiation image detecting device that is used in a radiation imaging apparatus, to detect a radiation image after conversion of the radiation image into an optical image.
2. Description Related to the Prior Art
A radiation imaging apparatus includes a radiation generating device for emitting radiation i.e. X-ray and a radiation image detecting device for detecting a radiation image that is formed by the radiation passed through a sample. As a type of radiation image detecting device, an indirect conversion type radiation image detecting device is known. The indirect conversion type radiation image detecting device, which has a photodetector and a scintillator stacked to each other, detects the radiation image after conversion of the radiation image into an optical image. The scintillator is a phosphor layer of CsI, GOS (Gd2O2S; Tb), or the like, and converts the radiation image into the optical image. The photodetector has an array of pixels each of which includes a photodiode (PD) and a thin film transistor (TFT) formed in a substrate. The photodetector converts light radiating from the scintillator into electric charge by each PD, and accumulates the electric charge on a pixel-by-pixel basis. This electric charge is read out via the TFT.
The indirect conversion type radiation image detecting device adopts either a PSS (penetration side sampling) method or an ISS (irradiation side sampling) method. In the PSS method, the scintillator and the photodetector are disposed in this order from a radiation incident side. Contrarily, in the ISS method, the photodetector and the scintillator are disposed in this order from the radiation incident side. In the ISS method, the scintillator converts the radiation passed through the photodetector into the light, and the PDs detect the light propagating in a direction opposite to a radiation incident direction. The scintillator emits the light by a larger amount on its radiation entrance side than on an opposite side (radiation exit side). Since the photodetector is opposed to the radiation entrance side of the scintillator in the ISS method, the ISS method has the advantage over the PSS method in sensitivity and image sharpness.
U.S. Pat. No. 8,049,177 corresponding to Japanese Patent Laid-Open Publication No. 2011-17683 discloses a radiation image detecting device of the ISS method in which a scintillator is formed of columnar crystals of CsI or the like on a substrate (support substrate). The scintillator is disposed such that tip ends of the columnar crystals are opposed to a photodetector. In this scintillator, light is produced in each columnar crystal upon application of the radiation, and propagates through the same columnar crystal by a light guide effect of the columnar crystal. The columnar crystals prevent dispersion of the light produced in the scintillator, so the sharpness of the radiation image is improved.
In this scintillator, a carbon plate, a CFRP (carbon fiber reinforced plastic) plate, a glass plate, a quartz plate, a sapphire plate, or a metal sheet made of iron, tin, chromium, aluminum, or the like is used as the substrate. Out of these materials, an aluminum sheet is most generally used as the substrate. However, aluminum has a high backscatter coefficient, in other words, scatters a large amount of the incident radiation in a return direction. Thus, in the radiation image detecting device of the ISS method, the radiation backscattered by the substrate causes unnecessary light emission in the scintillator. This light exits from the scintillator at a position away from an entrance position of the radiation, and therefore degrades the sharpness of the detected radiation image.
Japanese Patent Laid-Open Publication No. 2010-096616 proposes to provide on a substrate a radiation absorbing layer made of a material containing lead, tungsten, tantalum, or the like having an atomic number of 50 or more, in the radiation image detecting device of the ISS method. The radiation absorbing layer absorbs the radiation and reduces backscattering.
However, the provision of the radiation absorbing layer made of the material having the large atomic number to reduce the backscattering, as described above, and additionally, the formation of the columnar crystals on the radiation absorbing layer necessarily cause weight increase. Thus, this is unsuitable for a portable type radiation image detecting device such as an electronic cassette. Also, since the columnar crystals of CsI or the like deliquesce, the scintillator must be highly resistant to moisture.